The present invention relates to a video signal encoding apparatus for enlarging a video signal through a coding process.
In encoding images, such situations frequently occur in which it is required to enlarge moving pictures. When an input moving picture is encoded after it is enlarged, we consider that the picture is first enlarged in a spatial domain and then encoded. In this case, such processes as discrete cosine transform (DCT) are performed subsequent to the enlargement of the picture. Accordingly, the volume of calculation required for encoding increases as compared with that where the image is not enlarged.
A video signal encoding apparatus capable of encoding an input moving picture after enlarging it, while avoiding the problem just described of increase in the volume of calculation, is disclosed in Japanese Non-examined Publication No. H10-308946. An example of the video signal encoding apparatus performing encoding after enlargement of an input moving picture is shown in FIG. 5. This apparatus is described in the case of doubling an input picture in the vertical direction.
Block converter 502 divides input image 501 into 8xc3x974 blocks formed of eight horizontal pixels and four vertical lines. Inter-frame prediction unit 503 performs inter-frame prediction with the use of the divided 8xc3x974 blocks and reference picture generated by reference picture replacing unit 508 and generates 8xc3x974 differential blocks and motion vectors. When inter-frame prediction is not performed, 8xc3x974 blocks generated in block converter 502 comes to be 8xc3x974 differential blocks.
Then, 8xc3x974 DCT coefficient blocks are generated from 8xc3x974 differential blocks by DCT unit 504. Quantizer 505 quantizes 8xc3x974 DCT coefficient blocks to generate 8xc3x974 quantization coefficient blocks. Inverse quantizer 506 inverse-quantizes 8xc3x974 quantization coefficient blocks to generate 8xc3x974 inverse quantization coefficients blocks.
Further, inverse discrete cosine transform (IDCT) unit 508 performs IDCT processing on 8xc3x974 inverse quantization coefficient blocks to generate 8xc3x974 decoded blocks. Reference picture replacing unit 508 updates the reference picture with the use of 8xc3x974 decoded blocks and motion vectors generated in inter-frame prediction unit 503.
Vertical doubling unit 509 enlarges 8xc3x974 quantization coefficient blocks, obtained through quantization processing, in a frequency domain to generate 8xc3x978 quantization coefficient blocks. Thus, the size in the vertical direction of the block becomes double that of the input picture.
Variable length coding (VLC) unit 510 performs variable length coding processing on 8xc3x978 quantization coefficient blocks to generate compressed data 511.
By subjecting data first to DCT and quantization processing and then encoding the obtained data in a frequency domain as described in the above example, the amount of data calculation required for the picture coding is reduced from that where the input picture is first enlarged and then encoded.
In the example described above, the size of the block subjected to IDCT processing in the local decoder is 8xc3x974. On the other hand, when a decoding apparatus, which has received the output of the compressed data from this apparatus, decodes the compressed data, IDCT processing is performed on 8xc3x978 blocks as a unit. Namely, the size of the blocks subjected to IDCT processing in the former apparatus, i.e., the encoding apparatus for encoding an image, is different from the size of the blocks subjected to IDCT processing in the latter apparatus, the decoding apparatus receiving the encoded data and decoding the received data. Due to the difference in the size of blocks, errors occur in the frame subjected to the inter-frame predictive processing between the image encoded and the image decoded. Especially, errors accumulate at the edge portions of the image and this causes distortion of the image or color registration error.
The object of the present invention is the provision of a video signal encoding apparatus enlarging the image in a frequency domain when performing encoding of a motion picture thereby reducing the amount of data calculation required for the encoding and suppressing deterioration in image quality.
The video signal encoding apparatus of the present invention performs DCT calculation and quantization with the use of mxc3x97n block (m, n: natural number) as the unit block for coding a motion picture and, then, converting the block into sxc3x97t block (s, t: natural number, sxe2x89xa7m, txe2x89xa7n), performs variable length coding. Thus, the apparatus generates compressed data of an enlarged motion picture and delivers the enlarged output.
In the local decoder of the apparatus, it is selected by a switcher whether mxc3x97n inverse quantization coefficients, obtained by inverse quantization, are to be decoded by an sxc3x97t IDCT unit or by an mxc3x97n IDCT unit, in accordance with the accuracy in motion compensation.
Further, the apparatus has a resizing unit for adjusting a motion-compensated reference picture to the picture size of an input image.
In another embodiment of the invention, the apparatus has the same configuration as that of the above described embodiment except that the local decoder in this apparatus decodes mxc3x97n inverse quantization coefficients, obtained by inverse quantization, in an sxc3x97t IDCT unit.
Having the described configuration, the video signal encoding apparatus performs the encoding by enlarging a motion picture in a frequency domain so that the amount of data calculation in the encoding is decreased and deterioration in picture quality is suppressed.